This invention relates to adjusting the airflow in turbine components, e.g., gas turbine components, having airflow holes by depositing an overlay metallic coating. This invention further relates to the turbine component whose airflow has been adjusted by depositing such an overlay metallic coating.
Airflow holes are formed in many gas turbine components, such as combustor liners, for transporting film air through the component to typically cool the component and to form a fluid barrier between the component and hot gases traveling in the main flowpath of the engine. In addition to flowing air over the hot surfaces, combustor liner cooling is also provided by a thin layer of cooling air along the inner, combustion side of the liner by directing airflow through an array of very small airflow holes formed in the liner, typically having a diameter of from about 0.02 to about 0.03 inches (from about 508 to about 762 microns). This film cooling is also induced through “nugget” holes that are typically in one row along the forward edge of the liner, and a much greater number of “transpiration” holes that are typically arranged in a plurality of rows across the entire surface of the liner to induce a more uniform airflow. These “transpiration” holes are typically angled or slanted from the “cold” or air supply side, to the “hot” or combustion side of the liner in a downstream direction, and typically have a circumferential orientation. See, for example, FIG. 2 of commonly assigned U.S. Pat. No. 6,655,149 (Farmer et al), issued Dec. 2, 2003. This arrangement, commonly referred to as “multi-hole film cooling,” reduces the overall liner cooling airflow requirement because the mass flow through the airflow holes dilutes the hot combustion gas next to the liner surface, with the flow through the airflow holes providing convective cooling of the liner walls. In addition to these smaller diameter “nugget” and “transpiration” airflow holes, larger diameter holes (commonly referred to as “dilution holes”) to introduce dilution air into the combustion zone are also provided at spaced intervals. See, for example, commonly assigned FIG. 2 of U.S. Pat. No. 6,408,629 (Harris et al), issued Jun. 25, 2002 and FIG. 3 of U.S. Pat. No. 6,655,149 (Farmer et al), issued Dec. 2, 2003.
The combustion side of these combustion liners can be coated with a thermal barrier coating to help protect the liner from thermal fatigue caused by the hot gas radiation and conduction to the combustor liner or liners. See commonly assigned U.S. Pat. No. 6,620,457 (Farmer et al), issued Sep. 16, 2003, which discloses a physical vapor deposition process to thermally insulate the combustor liner. After a period of service, these combustor liners are typically removed from the engine for replacement, repair, cleaning and/or removal of contaminants (e.g., oxidative deposits and residual combustion products), cracking and other thermally induced stresses that the liners have been subjected to. At least some of these cracks run through the various holes, including the smaller “nugget” and “transpiration” holes.
To repair a part or component, or a portion of a part or component, adequate cleaning of the surface thereof is typically required. During cleaning, the thermal barrier coating and contaminants are typically removed from the combustor liners by chemical and/or mechanical processes, for example, a conventional acid strip process. Repair of the combustor liners, and in particular repair of the cracks that typically form in the liners during operation and use, can cause at least some of the very small “nugget” and “transpiration” holes to become obstructed, occluded, plugged, or otherwise blocked which can then require chemical and/or mechanical processes to reopen the holes. The reopening of these airflow holes, as well as the chemical stripping process that removes the coatings and contaminants, can also remove some of metal substrate of the combustor liner where these holes are located, resulting in enlarging of these holes. These enlarged airflow holes can significantly and undesirably increase the airflow of these liners. Indeed, after several cycles of such cleaning and repair, the airflow can be increased to the extent that the combustor liner is no longer usable.
Problems in controlling airflow can also occur during the original manufacture of the combustor liners. To form typically thousands of these very small “nugget” and “transpiration” holes, and especially at a slanted angle, the liner is typically drilled using special machining processes such as laser beam or electrical discharge machining (EDM) processes. While a certain amount of control can be exercised over the pattern and size of the drilled holes, it is still extremely difficult to provide combustor liners that have a consistent pattern and size of holes such that the airflow rate is within desired limits. In addition, laser or EDM drilling forms a recast layer along the surface of the hole as it is generated. During subsequent stripping and cleaning of the combustor liner, this recast layer can also be removed, thus enlarging the hole and increasing the airflow.
A method for adjusting airflow in a turbine component having such airflow holes is disclosed in commonly assigned U.S. Pat. No. 6,408,610 (Caldwell et al), issued Jun. 25, 2002. This method involves depositing a thermal barrier coating by a physical vapor deposition (PVD) process (e.g., electron beam PVD) on the exterior and/or interior surfaces of the component to at least partially obstruct the airflow through the airflow holes. While this method provides the ability to adjust the airflow through the airflow holes, the physical vapor deposition apparatus, because of its size, may not provide the flexibility needed to use it with some turbine components. In addition, thermal barrier coatings typically comprise ceramic materials that may or may not adhere adequately to the metal surface of the liner over time without an overlay metallic bond coat layer. See U.S. Pat. No. 6,620,457, supra, which discloses spraying NiCrAlY as a bond coat layer 110 of from about 4 to about 10 mils on the inner combustion surface 40 of the combustor liner 14 before depositing the thermal barrier coating 120.
Accordingly, it would be desirable to be able to economically and evenly adjust the airflow in turbine components (e.g., gas turbine components), such as combustor liners, having a large quantity of smaller diameter airflow holes where the air volume of the respective holes has been changed, and especially enlarged, during subsequent repair, replacement, cleaning, and/or removal processes. It would also be desirable to be able to adjust the airflow through turbine components after final manufacture without removing, or substantially removing, previously applied thermal barrier coatings. It would additionally be desirable to be able to adjust the airflow in turbine components, such as combustor liners, having airflow holes that, when originally manufactured (i.e., an OEM component), require airflow adjustment to be within acceptable limits. It would be further desirable to be able to have the flexibility to adjust the airflow in a variety of turbine components having airflow holes and especially a component having many thousands of airflow holes as, for example, a combustor liner.